Industrial filters, particularly honeycomb-type filters, are typically used for removing polluting particles from gases such as flue gas, engine exhausts, stacks, pneumatic transport and high temperature gases. Honeycomb filters are advantageously highly compact, structurally sound, self-supporting and lightweight. They are free of constraints of can velocity and they can be put close together in large arrays. Because of their compact size, they are generally washable, less prone to explosions from combustible dust and may generally operate in either horizontal or vertical position.
The traditional design of honeycomb filters comprises long parallel inlet and outlet channels open on alternate ends, arranged in either a checkerboard or a triangular pitch fashion and made of filter media. Gas flows from the inlet channels to the outlet channels, leaving an accumulation of solid particles along the filtering boundaries of the inlet channels which necessitates periodic cleaning of the filter.
The most common technique for cleaning filters is through flow reversal. Pressurized gas is projected backward in the filter, inversely to the gas flow of the normal filtering operation, dislodging the accumulated particles and pushing them out of the filter through its inlet.
Because of the great length of the channels and their small section, the axial velocities in the filter are very high compared to non-honeycomb filters. Because of the high tangential velocity in the channels during reverse flow cleaning, an important part of the cleaning is done tangentially by axial shear forces that preferably remove the surface of the filter cake.
The cake located deeper in the filter medium and hereby named as residual cake, is shielded from the high velocity gases that move along the channels. This residual cake will not be removed. When filtration re-starts, the residual cake acts as a filter medium and prevents particulate matter penetration through the filter as it happens in conventional filters due to over-cleaning.
A major problem in honeycomb filters is therefore solid particulate accumulation on and near the closed end of the inlet channels, which cannot be easily be removed through flow reversal. Referring to FIGS. 1A to 1D (PRIOR ART), the schematics of filtration inside a prior art honeycomb cleaned by flow reversal are illustrated, showing the problem of dead-end accumulation. FIG. 1A shows a typical filtration cycle: particle laden gases enter an inlet channel 4 through its open end 31, cross the filter media partition wall 18 into the contiguous outlet channel 5 and exit through the open end 32. The particles from the dirty gases are deposited as a filter cake 15 on the partition wall 18 inside the inlet channel 4.
FIG. 1B illustrates the cleaning schematics of conventional honeycombs. The reverse flow of gases enters the open end 32 and the outlet channel 5, crosses the partition wall of filter media 18 into the inlet channel 4 where it detaches and transports the filter cake 15 out through the open end 31. The low flow near the closed end 33 of the inlet channel 4 however is not strong enough to remove all of the cake 15, and some of it remains inside the inlet channel's dead-end 33.
FIG. 1C shows a second filtration cycle following the cleaning cycle of FIG. 1B. As can be seen, in the proximity of the dead-end 33, the new cake 15 is deposited over the leftover cake 15 from the earlier filtration cycle, creating an even thicker accumulation. This problem worsens over time, as may be seen on FIG. 1D, which shows the cake 15 accumulation on and near the dead-end 33 after a number of cleaning and filtration cycles.
Mechanisms other than flow reversal were thought of, including the in-place burning of the filtered particles in cases where it is combustible. In this respect, U.S. Pat. Nos. 4,276,066 (BLY et al.) and 4,346,557 (SHADMAN et al.) both disclose honeycomb filters for Diesel exhaust emissions composed mainly of soot particles, in which cleaning is accomplished by burning the soot, thereby eliminating the dead-end particle accumulation. It should be noted however that this approach works only if the filtered particles are combustible, ash-free and where the high temperatures developed during combustion can be tolerated.
In view of the above, there is therefore a need for an easily cleanable dust collecting filter which alleviates the above-mentioned drawbacks.